Top Banner

Click here to load reader

Experimental Research and CFD Simulation on Microchannel Evaporat

Sep 07, 2015

ReportDownload

Documents

study of Microchannel Evaporator

  • Purdue UniversityPurdue e-PubsInternational Refrigeration and Air ConditioningConference School of Mechanical Engineering

    2006

    Experimental Research and CFD Simulation onMicrochannel Evaporator Header to Improve HeatExchanger EfficiencyZhihai Gordon DongAmerican Power Conversion Corp.

    John BeanAmerican Power Conversion Corp.

    Follow this and additional works at: http://docs.lib.purdue.edu/iracc

    This document has been made available through Purdue e-Pubs, a service of the Purdue University Libraries. Please contact [email protected] foradditional information.Complete proceedings may be acquired in print and on CD-ROM directly from the Ray W. Herrick Laboratories at https://engineering.purdue.edu/Herrick/Events/orderlit.html

    Dong, Zhihai Gordon and Bean, John, "Experimental Research and CFD Simulation on Microchannel Evaporator Header to ImproveHeat Exchanger Efficiency" (2006). International Refrigeration and Air Conditioning Conference. Paper 753.http://docs.lib.purdue.edu/iracc/753

  • R079, Page 1

    International Refrigeration and Air Conditioning Conference at Purdue, July 17-20, 2006

    EXPERIMENTAL RESEARCH AND CFD SIMULATION ON MICROCHANNEL EVAPORATOR HEADER TO IMPROVE HEAT EXCHANGER EFFICIENCY

    Zhihai Gordon Dong1, John Bean Jr.2

    1Research and Development, NetworkAir Dept.,

    American Power Conversion Corp. 801 Corporate Center Dr., St. Charles, MO, US, 63304

    Email: [email protected]

    2Research and Development, NetworkAir Dept., American Power Conversion Corp.

    801 Corporate Center Dr., St. Charles, MO, US, 63304 Email: [email protected]

    ABSTRACT Microchannel evaporator performs superior heat transfer efficiency and capacity at compact size comparing with conventional tube-fin evaporator. Design an appropriate coil header assembly is one of major tasks, which could affect the desired heat exchanger efficiency and capacity. An experimental investigation of three coil inlet header configurations, which are single, dual and distributor inlet headers, is conducted in the paper. A practical evaluation method, by means of measuring air temperature differential across coil to evaluate refrigerant distribution uniformity in coil header, is introduced. CFD models are also generated to simulate refrigerant liquid flow contours of these three inlet header configurations. Finalize the research, the distributor header configuration appears the most uniform distribution in conjunction with symmetrical flow distribution. The dual inlet header configuration also significantly improves distribution uniformness and symmetry comparing with the single inlet header configuration.

    1. INTRODUCTION

    1.1 Introduction of microchannel heat exchanger A microchannel heat exchanger appears advanced characters comparing with a conventional round tube-fin heat exchanger. Refrigerant flows through multiple microchannel flat tubes, which contain microchannels ports rather than single wall round tubes. This significantly enhances the heat transfer area and overall film coefficient of the microchannel heat exchanger. High efficiency of the microchannel heat exchanger enables the heat exchanger to be made in smaller size, light weight, and yet has the same performance as a conventional round tube-fin heat exchanger. Refrigerant charge of the cooling system is also reduced. Due to many advantages of microchannel heat exchanger, it has been widely applied in residential air conditioning and automotive industry. However, comparing with a round tube-fin heat exchanger, microchannels ports causes higher refrigerant pressure drop across heat exchanger, it might be an issue to some systems. Condensation and defrost of microchannel coil are also major issues for refrigeration and air conditioning applications. The uniformity of refrigerant distribution within coil inlet header is another major issue for microchannel heat exchanger. A properly designed coil inlet header should uniformly distribute refrigerant into microchannels, and refrigerant would perform sufficient heat transfer inside microchannels tubes. Eventually, coil cooling efficiency is optimized and capacity is maximized. On the contrary, a defective coil header assembly could cause uneven refrigerant flow inside microchannels and reduce coil cooling efficiency and capacity. In the worst situation, the defective header design might cause the danger of gas and liquid flow separation, which could significantly damage coil heat transfer performance. Therefore, to achieve uniform refrigerant distribution, the coil header configuration and orientation becomes to be a very important design task.

  • R079, Page 2

    International Refrigeration and Air Conditioning Conference at Purdue, July 17-20, 2006

    1.2 A refrigerant liquid pumping cooling system for data center air conditioning application The major duty of a data center air conditioning system is to remove sensible heat, which is generated by electronic equipments. Condensation is controlled and minimized in the system. Ideally, sensible heat ratio of this air conditioning system would equal to one. Modern servers are integrated into limit space. Heat load density in the space is high. A compact air conditioning system with high cooling efficiency and capacity needs to be developed to remove this high density heat flux. A R134a liquid pumping cooling system is introduced to this application. The cycle of system circuit is shown in Figure (1).In the diagram, subcooled liquid R134a enters pump intake at state point (1) to process adiabatic compression. Further subcooled liquid R134a leaves pump discharge port at state point (2). State point (2) to (3) presents liquid R134a flows through liquid line and reaches to inlet of evaporator. From state point (3) to (4), subcooled liquid is vaporized in evaporator. Slight superheated gas leaves evaporator at state point (4) and return back to condenser inlet at state point (5) via a suction line. R134a gas rejects heat to chilled water in condenser, and it becomes subcooled liquid. The subcooled liquid returns back to pump intake at state point (1) and cycle starts over again.

    Figure (1) R134a liquid pumping cooling system cycle

    1.3 Microchannel heat exchanger in the refrigerant liquid pumping cooling system The microchannel coil is horizontally installed in space above two symmetrical heat loads. Upon means of control coil evaporating temperature, condensation is avoided to be generated on coil. However, due to subcooled liquid R134a flows into the large dimensional coil inlet header (approx 23 long), a proper coil inlet header configuration becomes important issue to accomplish refrigerant uniform distribution into microchannels. 1.4 Three configurations of microchannel coil inlet header To optimize coil inlet header assembly, three configurations of microchannel coil inlet header are conducted and investigated in this paper. These three configurations - single, dual and distributor inlet headers are illustrated in Figure (2).

    (a) Single inlet header (b) Dual inlet header (c) Distributor inlet header

    Figure (2) Three configurations of microchannel coil inlet header

    The single inlet header is constructed by a single round tube, which is simply welded to center of coil inlet manifold in perpendicular direction. The dual inlet header employs a secondary header, which is jointed to the coil inlet manifold via two round tubes at and length of coil inlet manifold. Main liquid entry line is connected to center of the secondary header. The entire dual inlet header assembly is furnace welded. Refrigerant liquid flows into the secondary header first. It is subsequently divided into two braches by the two round interjunction tubes, and flows into coil inlet manifold. The third configuration is the distributor inlet header. Refrigerant liquid flow is evenly divided inside hollow conical portion of the distributor. A group of small capillary tubes are connected to the

  • R079, Page 3

    International Refrigeration and Air Conditioning Conference at Purdue, July 17-20, 2006

    distributor to pick up the branched refrigerant flow. They deliver refrigerant liquid into coil inlet manifold at even portions. 2. THE APPROACH TO EVALUATE REFRIGERANT DISTRIBUTION UNIFORMITY

    INSIDE COIL INLET HEADER 2.1 Heat transfer and thermodynamic process inside microchannel tube To simplify heat transfer and thermodynamic analysis inside microchannel ports, we consider the group of microchannel ports as one single microchannel tube. The process is illustrated in Figure (3).

    Figure (3) Refrigerant heat transfer and thermodynamic process inside microchannel tube (Not to scale)

    As shown in Figure (3), refrigerant flow inside microchannel tube is composed by three segments, which are liquid sensible heating, evaporating, and gas sensible heating segments. The local convective heat transfer flow rate Qhx is calculated in Equation (1).

    Qhx = 0A h thx dA (1) The convective heat transfer coefficient h is strong dependent upon the refrigerants physical properties, situation and Reynolds number. It is much higher within evaporating (two-phase flow) segment due to bubble generation and other major factors. Heat transfer flux appears extremely active within this area, while the heat transfer density is much lower within subcooled and superheated segments (one-phase flow). There is no phase change within liquid sensible heating and gas sensible heating segments. Refrigerant sensible heat flow rate in these areas is donated in Equation (2). Qr = mr Cpr tr (2) Refrigerant performs phase change in evaporating area, the latent heat flow rate of refrigerant in the area is shown in Equation (3).